The present invention is directed to a system and method for processing images obtained through computed tomography (CT) or the like and more particularly to such a system and method in which artifacts caused by the presence of metal implants are reduced or eliminated.
X-ray computed tomography (CT) is an important imaging technique, particularly in medical and dental applications. A series of X-ray beams from many different angles are used to create cross-sectional images of the patient""s body. Those images from multiple slices are assembled in a computer into a three-dimensional picture that can display organs, bones, and tissues in great detail. CT offers high spatial resolution, three-dimensional registration and minimal blurring caused by motion.
However, the presence of strongly attenuating objects, such as metal implants or fillings, causes streak artifacts, also called starburst artifacts, in the image. Another problem encountered in CT is inhomogeneous estimation of tissue density caused by inhomogeneous illumination. While much investigation has been done into reducing or eliminating those problems, a satisfactory technique has not yet been found.
One attempt at a solution to the problem of artifacts caused by metal implants is found in U.S. Pat. No. 5,561,695 to Hu. The patent teaches a method for improving CT image quality in which data from a helical reconstruction are separated into a background component and a sharp-structure component. The separation can be performed using gray-scale thresholding, since the sharp structures and the image background usually have widely differing CT numbers. The image background is filtered to remove high-frequency artifacts. The images are recombined. However, the technique of Hu introduces more computational complexity than is desired, and the Hu approach does not appear to have been widely adopted by the radiology community.
It will be readily apparent from the above that a need exists in the art for a computationally efficient technique for reducing or eliminating streak artifacts and illumination inhomogeneity.
It is therefore an object of the present invention to reduce or eliminate streak artifacts caused by metal implants or the like.
It is another object of the present invention to correct illumination inhomogeneities.
It is still another object of the present invention to achieve the above objects in a computationally efficient way.
To achieve that and other objects, the present invention is directed to a technique for CT or other imaging that works directly with the reconstructed data. The technique has two basic steps: 1) illumination correction and 2) adaptive 3D filtering. The algorithm starts by estimating the direction of the streak and the degree of inhomogeneous densities by gray scale morphology dilation. Then, it proceeds to estimated the correct densities based on the estimations and to reduce the streak by an adaptive 3D filtering whose parameters depend on the streak direction and the local image contrast.